there are four ways to import library to php, they are include, include_once, require and require_once.

what is the difference?

  • require

when the file is required by your application, e.g. an important message template or a file containing configuration variables without which the app would break.

  • require_once

when the file contains content that would produce an error on subsequent inclusion, e.g. function important() { /* important code */} is definitely needed in your application but since functions cannot be redeclared should not be included again.

  • include

when the file is not required and application flow should continue when not found, e.g. great for templates referencing variables from the current scope or something

  • include_once

optional dependencies that would produce errors on subsequent loading or maybe remote file inclusion that you do not want to happen twice due to the HTTP overhead

credit to Tarek Kalaji

According to others suggestions, use require_once 99.9% of the time.